Method for producing febrous carbon structures

ABSTRACT

Carbonized fibrous material of improved resiliency and tensile strength is produced by impregnating rayon fibers with a solution of polyethylene or polypropylene dissolved in a volatile hydrocarbon solvent such as xylene, driving off the solvent, and thereafter heating the impregnated fibers to a temperature adequate to carbonize the rayon. After carbonization, the polyethylene-treated fibers showed a marked improvement in resiliency as well as an average tensile strength increase of about 55 percent over untreated fibers of the same type.

United States Patent 6 Claims, No Drawings US. Cl 23 2094, 8/115.6,8/116, 23/2092, 23/2094, 117/26, 117/46, 117/145, 264/29 Int. Cl C0lb31/07 Field of Search 23/2091, 209.2, 209.4; 117/26,46,145;8/115.5,1l5.6,116

References Cited UNITED STATES PATENTS 3,116,975 1/1964 Cross et a1.23/2091 inventor Charles R. Schmitt Oak Ridge, Tenn.

Appl. No. 8,695

Filed Feb. 4, 1970 Patented Sept. 2 1, 1971 Assignee The United Statesof America as represented by the United States Atomic Energy CommissionMETHOD FOR PRODUCING FIBROUS CARBON STRUCTURES 3,121,698 2/1964Orsinoetal. l17/l45X 3,281,261 10/1966 Lynch l17/46 3,294,489 12/1966Millington et a1... 23/2094 3,305,315 2/1967 Bacon et a1. 23/20913,395,970 8/1968 Machell 8/1 16.2

Primary Examiner-Edward J. Meros AuomeyRoland A. Anderson ABSTRACT:Carbonized fibrous material of improved resiliency and tensile strengthis produced by impregnating rayon fibers with a solution of polyethyleneor polypropylene dissolved in a volatile hydrocarbon solvent such asxylene,

driving off the solvent, and thereafter heating the impregnated METHODFOR PRODUCING FIBROUS CARBON STRUCTURES The present invention relatesgenerally to the preparation of fibrous carbon products and moreparticularly to a method of treating rayon fibers with an organicpolymer of polyethylene or polypropylene to improve the resiliency andtensile strength of the fibers when the latter are converted to acarbonaceous state. This invention was made in the course of, or under,a contract with the U. S. Atomic Energy Commission.

Textile materials such as provided by the weaving or knitting ofcellulosic fibers with yarn, fabrics, braids, felts, or the like, aresuitable structural materials because of their high tensile strength andflexibility. Conversion of these cellulosic materials to a carbonaceousstate provides some additional features in that the well known chemicaland physical properties of carbon can be used advantageously in manystructural applications. However, while gaining such properties asattributed by the carbon, the carbonized material suffers considerablelosses in the areas of tensile strength and resiliency so as tosignificantly detract from its usage in many applications where suchphysical characteristics are desired.

Accordingly, it is the aim and principal object of the present inventionto overcome or substantially minimize the above and other shortcomingsor drawbacks suffered by the carbonized cellulosic material as preparedby practicing previously known techniques. This goal is achieved byemploying a method of preparing carbonized cellulosic material wherebythe tensile strength and resiliency are significantly improved overthose previously provided.

Broadly, the method of the present invention comprises the steps ofimpregnating or contacting fibers of regenerated cellulose, i.e., rayon,with an organic polymer of polyethylene or polypropylene dissolved in asuitable hydrocarbon solvent therefor, forming the impregnated fibersinto the desired configuration, driving the solvent from the fibers suchas by heating to a temperature below the melting point of the organicpolymer but above the boiling point of the solvent, and thereafterheating the impregnated fibers to a temperature sufficient to convertthe regenerated cellulose to carbon. The presence of the organic polymerduring the pyrolysis or carbonization step apparently alters the surfaceof the fibers as well as the carbonized microstructure of the latter soas to provide the desirable tensile strength improvement which may be ashigh as about 55 percent over untreated carbonized fibers of the sametype. In addition to the significant increase in tensile strength, thefibers also show unexpected resiliency which greatly exceeds thatprovided by untreated carbonized rayon fibers.

In practicing the present invention the fibers are formed of regeneratedcellulose or rayon. Viscose rayon is the preferred type of rayon becauseof its ready availability and desirable physical properties, but othertypes of rayon such as cuprammonium rayon and saponified cellulose esterrayon may be satisfactorily employed as the fiber material. Naturalhigh-cellulose-content fibers such as cotton and other flosses are notsuitable for the purpose of this invention due to their weak carbonizedstrength. As briefly mentioned above, the regenerated cellulose fibersmay be in any desired form such as long or chopped monofilaments, yarn,braids, fabrics, or the like, prepared by any well-known knitting orweaving procedure.

In order to provide the regenerated cellulose fibers with the improvedresiliency and tensile strength characteristics upon carbonization, theyare treated, i.e., impregnated, with polyethylene or polypropylene whichhas been dissolved in a suitable hydrocarbon solvent, as will bediscussed below. Polyethylene is the preferred organic polymer used inthe treatment of the fibers since improvements in resiliency and tensilestrength are somewhat greater than provided by polypropylene. However,polypropylene, which is a linear hydrocarbon polymer containing littleor no unsaturation and has properties similar in many respects topolyethylene, provides carbonized rayon fibers with tensile strengthsgreater than provided by untreated carbonized rayon and a resiliencyfactor greater than provided by treating the fibers with other organicmaterials, as will be discussed in detail below.

Polyethylene is basically a polymer of ethylene produced by the reactionN(C H (C I-I )n where N in the plastic grade is in the range of about600 to 4,000. Polyethylene is a highly crystalline oriented material thecrystallization of which is improved by stretching and further improvedby annealing. The type polyethylene preferred in the subject applicationis the so-called low-density polyethylene, that is, about 0.91 or 0.92gram per cubic centimeter (g./cc.) at 25 C. The polyethylenescharacteristic of this group are preferred due to their relatively lowsoftening points, ease in handling, dissolution by solvents, and by thefact that the polyethylenes in this range do not solidify uponvolatilization of the solvent at a temperature which would bedetrimental to the pyrolysis of the rayon. The polyethylenes in thisrange normally have a melting point of about l00l 10 C., which issatisfactory for this application. Normally, these polyethylenes have amolecular weight of about 2,000 to 20,000. Preferably, the polyethylenehas a softening point of C., a density of approximately 0.91 g./cc. at25 C., and a molecular weight of about 7,000.

Without being held to a specific theory as to why the presence ofpolyethylene and, to some extent, polypropylene have significant andpositive effects on the quality of a carbon derived from rayon, thefollowing postulations are offered. It is believed that thepolyethylene-impregnated microstructure, after carbonizing, may containoriented particles of carbon deposited in the spinneret holes in thefibers. The surface of the carbon yarn is apparently altered by thepresence of polyethylene during pyrolysis, but merely providing asurface coating of carbon would not tend to have the profound effects onstrength and flexibility that were observed for the polyethylene-treatedproducts. It was also shown from experimental data that the polyethylenewas probably present as a fluid during pyrolysis of the rayon and waslater lost through volatilization. The influence of the polyethylene wasapparent, not only from the increased quality of the product, but alsofrom changes in the composition of pyrolysis products and changes in thesurface of the resulting carbon yarn. For example, a comparative studyof the rate of gas release from polyethylene-impregnated andunimpregnated rayon during pyrolysis has shown that a high release rateof carbon monoxide and carbon dioxide occurs for unimpregnated rayon at300 C. The corresponding gas release rates obtained for thepolyethylene-impregnated rayon were considerably lower, suggesting thatpolyethylene can react with rayon to inhibit the release of carbonmonoxide and carbon dioxide during partial pyrolysis at 300 C. It isbelieved that the pyrolysis of the rayon yarn in the presence of thepolyethylene resulted in the formation of a carbonaceous intermediateconsisting of chains of four carbons each. Thus, the similarity in thebasic structure of this intermediate and polyethylene could result ininteraction and incorporation of polyethylene into the structure.Another alternative to the particular mechanism possibly causing theincreased resiliency and tensile strength of the carbonized rayon isbelieved to be that the polyethylene actually altered the pyrolysismechanism by acting as a source of radicals and therefore contributedlittle mass product. No significant mass effects have been observed forthe treated rayon, and polyethylene alone leaves no char. Polypropyleneis believed to function in a manner somewhat similar to thepolyethylene, but the appearance of the carbonized yarns is different inthat the parallel lines which are formed along the rayon fibers arevisible with the polypropylene-treated yarns after carbonization,whereas the lines are substantially unobservable with thepolyethylene-treated fibers. Even so, the lines along the fibers of thepolypropylene-treated yarns are somewhat less evident than with theuntreated yarns after carbonization.

In order to treat the fibers with the polyethylene or the polypropyleneit is necessary that these organic polymers be in a solution which willfacilitate the impregnation of the fibers.

The formation of a polyethylene or polypropylene solution is readilyachieved by using a suitable hydrocarbon solvent such as xylene,toluene, or benzene. The dissolution of these organic polymers isnormally achieved at an elevated temperature, e.g., 60 C. or higher,since the organic polymers are relatively insoluble at lowertemperatures even in the particular solvents described.

it has been found that the quantity of the organic polymers in thesolution adequate to provide the rayon fibers with the necessaryquantity of impregnant is usually about 5-10 weight percent. Employmentof more than about weight percent does not appear to be advantageous inthat sufficient polyethylene or polypropylene is incorporated in thefibers using the weaker solutions. Further, the use of such weakersolutions assures that greater penetration into the fibers will beaccomplished. Less than about 5 weight percent does not appear to beadequate since insufficient quantities of the polymer penetrate thefibers to accomplished the the necessary property changes duringpyrolysis.

Rayon fibers in the form of fabrics, cloths, weaves, yarns,monofilaments, etc., are impregnated with the polymer by immersing thefibers into a bath consisting of the dissolved polymer. The impregnationis usually accomplished within a relatively short period of about 30minutes. increased penetration of the fibers may be achieved byemploying well-known pressure impregnation techniques. Upon completingthe impregnation of the fibers they are removed from the bath and formedinto the desired configuration. The fibers are then heated to atemperature below the melting point of the polymer to drive ofi thevolatile solvent. This driving off of the volatile solvent is usuallyachieved at a temperature less than about l00 C. with the solventslisted above. Upon completion of the removal of the solvent the fibersare heated in an inert atmosphere, e.g., argon, to a temperaturesufficient to convert the regenerated cellulose to a carbonaceous state.Usually, a temperature of about 900-l ,000" C. is adequate forpyrolysis. This temperature is preferably maintained for a duration ofabout 60 hours to assure complete conversion of the fibers to carbon.Exhausting the gases generated during the carbonization step isimportant since insufficient gas removal will result in inferiorproducts. If desired, the carbonized rayon may be converted to graphiteby heating the carbonized rayon in an inert atmosphere to a temperaturein the range of about 2,500 to 3,000 C.

Regenerated cellulose fibers, when treated and carbonized in accordancewith the teachings of the present invention, demonstrate a remarkableimprovement in the resiliency over fibers which have not been treated orwhich have been treated with other chemically impregnating plastics. inestablishing the improvement in the resiliency of the fibers produced bythe impregnation with polyethylene or polypropylene, carbon cones wereprepared by chemically impregnating rayon velvet with polypropylene,polyethylene, polystyrene diluted in xylene solvent, and formamide,acetic acid, tributylamine, and furfuryl alcohol in a partiallypolymerized state. After carbonizing the impregnated cones the resultingcarbon cones had dimensions of 1% inches in height and 1.3 to 1.5 inchesin diameter. These cones were subjected to a compressive load at theapex of the cone at a constant strain rate of 0.05 inch per minute. Thedata resulting from this testing of the cones impregnated by variouspolymers are shown in the table below. The data set forth for the coneimpregnated with polypropylene are derived from the testing of a largercone.

TABLE.COMPARATIVE RESILIENCY 0F CARBONIZED RAYON VELVET (AFTER VARIOUSCHEMICAL TREAT- MENTS PRIOR TO COKING) Specimen was very ..-.resi snTable Continued The polyethylene-treated carbonized rayon velvet had byfar the highest resiliency in that no fracture occurred, even aftercompletely flattening the apex of the cone. Polypropylene, while notproviding the resiliency afforded by the polyethylene, did show greaterresiliency than the other carbonized specimens, as is evident uponviewing the data in the above table. The use of long-chained aliphatichydrocarbons as impregnants had no effect on the carbon products withrespect to improved resiliency or tensile strengths. The vapor pressureof these hydrocarbon materials indicated a probable volatilization priorto pyrolysis of the yarn. Resins that were found to be undesirableincluded indene, vinyl indene, divinylbenzene, and polystyrene.Polystyrene is reported in the above table. All the products produced bythese last-mentioned resins were very weak and brittle.

The tensile strength of the polyethylene-treated rayon is significantlygreater before and after coking than that of untreated rayon. Randomsamplings of untreated rayon yarn 0.024 inch in diameter andpolyethylene-impregnated yarn 0.020 inch in diameter provided averagetensile strengths of 9,095 p.s.i. and 14,081 psi, respectively. Similarsamplings of carbonized untreated rayon yarn 0.016 inch in diameter andcarbonized yarn 0.014 inch in diameter which had been impregnated withpolyethylene provided average tensile strengths of 263 psi,respectively. The above data demonstrate that the carbonized rayon yarnshave an increased tensile strength of about 75 percent over theuntreated rayon yarn, and the polyethylene-impregnated rayon yarn showedabout a 55 percent improvement in tensile strength over the untreatedrayon yarn upon carbonization. Comparative tensile strength data onpolyethylene-impregnated monofilaments of rayon having a diameter ofapproximately 10 microns after carbonizing have shown average tensilestrengths of approximately 70,000 psi. as compared to average tensilestrengths of only approximately 50,000 psi. after carbonizing untreatedrayon monofilaments of the same diameter.

Flexible fabrics, yarns, fibers, felts, and the like, of carbonpresently have widespread industrial use and the added resiliency andtensile strength afforded to these carbonaceous materials will greatlyimprove their use in thermal and electrical applications, particularlywhere high temperatures are involved.

What is claimed is:

1. A method of producing a resilient carbonaceous fiber comprising thesteps of contacting a fiber of regenerated cellulose with a solutionconsisting essentially of an organic polymer selected from the groupconsisting of polyethylene and polypropylene and dissolved in a solventtherefor volatile at a temperature less than the melting temperature ofthe organic polymer, removing the fiber from the solution, heating thefiber to a temperature sufficient volatilize the solvent, and thereafterfurther heating the fiber in an inert atmosphere to a temperaturesufficient to convert the cellulose to carbon.

2. The method of producing a resilient carbonaceous fiber as claimed inclaim 1, wherein the solution consists essentially of about 5 to weightpercent of the organic polymer with the remainder being provided by thesolvent.

3. The method of producing a resilient carbonaceous fiber as claimed inclaim 1, wherein the fiber of regenerated cellulose is selected from theclass consisting of viscose rayon, cupraammonium rayon, and saponifiedacetate rayon, and wherein the temperature sufficient to convert thefiber to carbon is in the range of about 900 to 1,000 C.

4. The method of producing a resilient carbonaceous fiber as claimed inclaim 3, wherein the solvent is a hydrocarbon solvent selected from thegroup consisting of xylene, toluene, and benzene, and wherein thetemperature sufficient to 20,000, and a density of about 0.91 gram percubic centimeter.

2. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the solution consists essentially of about 5 to 10 weight percent of the organic polymer with the remainder being provided by the solvent.
 3. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the fiber of regenerated cellulose is selected from the class consisting of viscose rayon, cupraammonium rayon, and saponified acetate rayon, and wherein the temperature sufficient to convert the fiber to carbon is in the range of about 900* to 1,000* C.
 4. The method of producing a resilient carbonaceous fiber as claimed in claim 3, wherein the solvent is a hydrocarbon solvent selected from the group consisting of xylene, toluene, and benzene, and wherein the temperature sufficient to volatilize the solvent is less than about 100* C.
 5. The method of producing a resilient carbonaceous fiber as claimed in claim 3, including the additional step of heating the carbonized fiber to a temperature in the range of about 2,500* to 3,000* C. for converting the carbon to graphite.
 6. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the polymer is polyethylene having a melting temperature in the range of about 100* to 110* C., a molecular weight in the range of about 2,000 to 20,000, and a density of about 0.91 gram per cubic centimeter. 